Sports racquets (e.g., tennis rackets, squash racquets, badminton racquets, racquetball racquets) include a frame with a head portion. Strings are strung across the head portion of the frame to form a string bed. The head portion surrounds and defines the string bed. During play (e.g., tennis, squash, badminton, racquetball), the string bed is designed to contact and rebound a game piece such as a shuttlecock, racquetball or tennis ball.
Traditionally, the frames of sports racquets were made of wood. More recently, the frames have been made of extruded aluminum and composite materials.
Extruded racquet frames typically consist of aluminum alloy materials. Due to the ease of manufacturing these extruded structures, extruded racquet frames are preferred when producing low-price, mass-production frames. However, extruded aluminum racquet frames have many limitations, mostly due to the extruded process itself. These frame structures cannot be manipulated to increase strength, stiffness, or to change the extruded shape to create variable size frame section or wall thickness variability. Additionally, aluminum alloys are heavy and lack strength when compared to advanced plastics products used in today's industry.
Currently, composite racquet frames are the preferred type of racquet frames by most racquet sports enthusiasts, mostly because of the high strength-to-weight ratio in composite racquet frames. A bladder and cavity molding process is the preferred method used in today's manufacturing process of composite racquet frames. Using bladder molding allows for additional customization of the racquet frame. Combining materials, such as, carbon, Kevlar, fiberglass, boron, and other fibrous materials, are used to create structures that can vary in strength, rigidity, and weight. The freedom of controlling the fibers within the racquet frame structures has advanced racquet sports in recent years. It also has allowed for racquets to become more rigid, lighter, and larger, thus improving the player's ability and advancing the evolution of each individual sport.
Bladder molding a racquet frame is a process where structure is created by using compressed air, chemical reactions to increase pressure, or hot gases to apply internal pressure within the structure, thus forcing the material to the predetermined edges of the mold shape. At the same time, when pressure is added to the structure, the mold and the part is heated to a temperature which that accelerates the catalyst process to harden the racquet structure. Once hardened, a rough cured racquet frame is created.
The first step in bladder molding is to prepare the part for the molding process. This step is called creating a “pre-shape.” A pre-shape is a straight tube structure where later the part will be bent and formed into a shape that fits within the mold, known in the industry as a “hair pin”. The pre-shape process first begins with the use of a rigid mandrel. The rigid mandrel is used to create a predetermined cross sectional shape. A nylon bladder then is placed over the mandrel. This nylon bladder is sealed to contain the air, chemical or hot gas pressure. Now having a rigid mandrel with the bladder in place a lay-up process begins. The lay-up process is the application of multiple plies of carbon, Kevlar, fiberglass, etc. along the mandrel. Once the lay-up is completed, the pre-shape is placed into a mold having a special design. The mold is closed, and air is supplied to the bladder, forcing the material to the predetermined edges of the mold shape. Simultaneously, the mold and the part is heated to a temperature that accelerates the catalyst process to harden the racquet structure. Once hardened, a rough cured racquet frame is created.
A problem with bladder molding is that it is designed to create a hollow racquet frame structure. Although this decreases the overall weight of the frame, there is a sacrifice in the dynamic strength and durability of the frame.
Another problem with bladder molding is that holes still need to be drilled through substantially all of the frame to attach the strings to the frame. Drilling holes through the frame cuts the fibers of the composite frame material, weakening the frame structure.